Generic factory pattern in Typescript - Step-by-Step Execution

Concept Flow - Generic factory pattern

Call factory with type T

↓

Create instance of T

↓

Return new instance

↓

Use instance in code

The factory receives a type and creates a new instance of that type, then returns it for use.

Execution Sample

Typescript

class Car {
  drive() { return 'Driving'; }
}

function factory<T>(c: new () => T): T {
  return new c();
}

const car = factory(Car);
console.log(car.drive());

This code defines a factory function that creates an instance of a class passed as a type and constructor, then calls a method on the created instance.

Execution Table

Step	Action	Evaluation	Result
1	Call factory with Car	factory<Car>(Car)	Enter factory function
2	Create new instance	new Car()	Car instance created
3	Return instance	return new Car()	Car instance returned
4	Assign to variable	const car = factory(Car)	car holds Car instance
5	Call method	car.drive()	'Driving' returned
6	Output result	console.log(car.drive())	Prints: Driving

💡 Execution ends after printing 'Driving' to console.

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
car	undefined	Car instance created	Car instance assigned	Car instance with drive method

Key Moments - 3 Insights

Why do we use 'new c()' inside the factory?

Because 'c' is a constructor function passed to the factory, 'new c()' creates a new instance of the class. See execution_table step 2.

How does TypeScript know what type 'car' is?

The factory function uses a generic type 'T' and the constructor signature 'new () => T', so when called with 'Car', TypeScript infers 'car' is of type 'Car'. See execution_table step 1 and 4.

What if the class constructor needs parameters?

This simple factory expects constructors with no parameters. To handle parameters, the factory signature and call must be adjusted. This example shows the basic pattern. See code sample.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of step 3?

A The factory function is called

B A new Car instance is returned

C The drive method is executed

D The console prints 'Driving'

💡 Hint

Check the 'Result' column in execution_table row for step 3.

At which step is the variable 'car' assigned a value?

A Step 4

B Step 2

C Step 5

D Step 6

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table for assignment.

If the factory function did not use 'new', what would happen?

A It would create a new instance anyway

B It would throw a syntax error

C It would return the class itself, not an instance

D It would call the drive method automatically

💡 Hint

Recall that 'new' creates an instance; without it, the class constructor is not called.

Concept Snapshot

Generic factory pattern in TypeScript:
- Use a generic function with a constructor signature: function factory<T>(c: new () => T): T
- Inside, create instance with 'new c()'
- Returns a new instance of type T
- Allows creating objects without knowing exact class at compile time
- Works for classes with no-argument constructors

Full Transcript

This example shows a generic factory pattern in TypeScript. The factory function takes a class constructor as input and returns a new instance of that class. The flow starts by calling the factory with a class type, then inside the factory, it creates a new instance using 'new c()'. The instance is returned and assigned to a variable. Finally, a method on the instance is called and its result is printed. Variables change from undefined to holding the new instance. Key points include understanding the use of 'new' with a constructor, how TypeScript infers types with generics, and the limitation to no-argument constructors in this simple pattern.

Practice

1.

What is the main purpose of the Generic factory pattern in TypeScript?

easy

A. To enforce strict typing on all variables.

B. To create objects without specifying their exact class.

C. To handle asynchronous operations automatically.

D. To optimize code for faster execution.

Solution

1. Step 1: Understand the pattern's goal

   The generic factory pattern is designed to create objects without needing to know their exact class ahead of time.

2. Step 2: Compare options with the pattern's purpose

   To create objects without specifying their exact class. correctly states this purpose. Other options describe unrelated features.

3. Final Answer:

   To create objects without specifying their exact class. -> Option B

4. Quick Check:

   Generic factory pattern = create objects flexibly [OK]

Hint: Focus on object creation without class details [OK]

Common Mistakes:

• Confusing with async handling
• Thinking it enforces variable typing
• Assuming it optimizes runtime speed

2.

Which of the following is the correct syntax to declare a generic factory function in TypeScript?

function create<T>(): T { /* ... */ }

easy

A. function create<T>(): T { return new T(); }

B. function create<T>(): T { return T(); }

C. function create<T>(): T { return new Object(); }

D. function create<T>(): T { return {} as T; }

Solution

1. Step 1: Analyze generic function syntax

   The function declares a generic type T and returns an object of type T.

2. Step 2: Check return statements for correctness

   function create<T>(): T { return {} as T; } uses a type assertion {} as T which is valid to create a generic placeholder object. function create<T>(): T { return new T(); } tries to use new T() which is invalid because T is a type parameter, not a constructor. function create<T>(): T { return T(); } calls T() like a function which is invalid. function create<T>(): T { return new Object(); } returns a generic object but not typed as T.

3. Final Answer:

   function create<T>(): T { return {} as T; } -> Option D

4. Quick Check:

   Generic factory syntax = use type assertion [OK]

Hint: Use type assertion to return generic type instance [OK]

Common Mistakes:

• Trying to instantiate generic type with new
• Calling generic type as a function
• Returning untyped objects without assertion

3.

What will be the output of this TypeScript code using a generic factory pattern?

interface Animal { speak(): string; }
class Dog implements Animal { speak() { return "Woof"; } }
function createAnimal<T extends Animal>(ctor: new () => T): T {
  return new ctor();
}
const dog = createAnimal(Dog);
console.log(dog.speak());

medium

A. "Woof"

B. undefined

C. TypeError at runtime

D. Compilation error

Solution

1. Step 1: Understand the factory function behavior

   The function createAnimal takes a constructor ctor and returns a new instance of type T which extends Animal.

2. Step 2: Trace the code execution

   We pass Dog class to createAnimal, so it returns a new Dog instance. Calling dog.speak() returns "Woof".

3. Final Answer:

   "Woof" -> Option A

4. Quick Check:

   Generic factory creates Dog, speak() = "Woof" [OK]

Hint: Trace constructor call and method output [OK]

Common Mistakes:

• Assuming generic type can't be instantiated
• Confusing type constraints with runtime errors
• Expecting undefined instead of method return

4.

Identify the error in this generic factory function and select the fix.

function createInstance<T>(): T {
  return new T();
}

medium

A. Add a constructor parameter: ctor: new () => T and return new ctor().

B. Remove new keyword and call T() directly.

C. Change return type to any to avoid errors.

D. Use Object.create(T) instead of new T().

Solution

1. Step 1: Identify why new T() causes error

   TypeScript does not allow new T() because T is a type parameter, not a constructor function.

2. Step 2: Apply correct fix using constructor parameter

   Passing a constructor function ctor: new () => T allows calling new ctor() to create an instance of T.

3. Final Answer:

   Add a constructor parameter: ctor: new () => T and return new ctor(). -> Option A

4. Quick Check:

   Generic factory needs constructor parameter [OK]

Hint: Pass constructor as parameter to instantiate generic type [OK]

Common Mistakes:

• Trying to instantiate generic type directly
• Removing new keyword incorrectly
• Changing return type to any loses type safety

5.

You want to create a generic factory that can produce different shapes with a draw() method. Which approach correctly uses the generic factory pattern to support adding new shapes easily?

interface Shape { draw(): void; }
class Circle implements Shape { draw() { console.log("Circle"); } }
class Square implements Shape { draw() { console.log("Square"); } }
// What is the best factory implementation?

hard

A. function createShape<T>(): T { return {} as T; }

B. function createShape(): Shape { return new Circle(); }

C. function createShape(ctor: new () => T): T { return new ctor(); }

D. function createShape(type: string): Shape { if(type === 'circle') return new Circle(); else return new Square(); }

Solution

1. Step 1: Understand the requirement for flexibility

   The factory should create any shape implementing Shape without hardcoding types.

2. Step 2: Evaluate each option

   function createShape(ctor: new () => T): T { return new ctor(); } uses a generic constructor parameter allowing easy addition of new shapes. function createShape(): Shape { return new Circle(); } always returns Circle, no flexibility. function createShape(type: string): Shape { if(type === 'circle') return new Circle(); else return new Square(); } uses string checks, less type-safe and harder to extend. function createShape<T>(): T { return {} as T; } returns an empty object cast to T, which is unsafe.

3. Final Answer:

   function createShape<T extends Shape>(ctor: new () => T): T { return new ctor(); } -> Option C

4. Quick Check:

   Generic factory with constructor param = flexible shape creation [OK]

Hint: Use generic constructor param for flexible object creation [OK]

Common Mistakes:

• Hardcoding return types limits extensibility
• Using string type checks reduces type safety
• Casting empty objects loses real behavior